Conventional position & proximity sensing technologies require benign environments—and/or transmitter/receiver pairs (photo sensors, for example). Such techniques may be constrained to pass the object between two parallel elements, such as, a receiver/transmitter pair. Several types of sensors have traditionally been used for various forms of object detection, including optical sensors, color sensors, touch sensors, ultrasonic sensors, infrared sensors, and Sonar and laser sensors.
Light sensors may be included in the proximity sensor category, and are simple sensors that change the voltage of photoresistors or photovoltaic cells in accordance with the amount of light detected. Light sensors may be, for example, used in very popular applications for autonomous robots that track a marked path.
With color sensors, different colors are reflected with different intensity, for example, an orange color reflects red light in an amount greater than a green color. Color sensors are in the same general category as light sensors, but with a few extra features that can be useful for applications where it is necessary to detect the presence of an object with a certain color or to detect the types of objects on surfaces.
Touch sensors may be included in the proximity sensors category and are designed to sense objects at a small distance with or without direct contact. These sensors are designed to detect changes in capacitance between the onboard electrodes and an object.
Ultrasonic sensors are designed to generate high frequency sound waves and receive an echo reflected by an object. These sensors are used in a wide range of applications and are very useful when it is not important to detect colors, surface textures, or transparency. Ultrasonic sensors may have the following advantages: the output value is linear with the distance between the sensor and the target, the sensor response is not dependent on the colors, transparency of objects, optical reflection properties, or by the surface texture of the object, they are designed for contact-free detection, sensors with digital (ON/OFF) outputs have excellent repeat sensing accuracy, they provide accurate detection of even small objects, and they may work in critical conditions such as dirt and dust. However, ultrasonic sensors may have the following disadvantages: they must view a high density surface for good results (e.g., a soft surface such as foam and cloth has low density and may absorb sound waves emitted by the sensor, they could experience false detection if some loud noises are received, they have a response time slightly less than other types of sensors, they have a minimum sensing distance, and some changes in the environment may affect the response of the sensor (temperature, humidity, pressure, etc.).
Infrared sensors measure infrared (IR) light that is transmitted in the environment to find objects by an IR light-emitting diode (LED). This type of sensor is very popular in navigation for object avoidance, distance measurements, or line following applications. IR sensors are very sensitive to IR lights and sunlight, which makes them useful for applications requiring great precision in spaces with low light. IR sensors may have the following advantages: they may detect infrared light over large areas, they may operate in real-time, they use non-visible light for detection, and they are inexpensive. Disadvantages of IR sensors is they are inherently very sensitive to IR lights and sunlight while be weak in sensing objects of darker colors such as black.
Sonar sensors maybe used primarily in navigation for object detection, even for small objects. These sensors have high performance on the ground and in water. Laser sensors may be very useful for tracking and detection targets located at a long distances. The distance between sensor and target is measured by calculating the speed of light and the time to receive a return. Laser sensors are very precise in measurement.